<meta charset="UTF-8">
<script src="jquery-3.2.1.js"></script>
<script>
$(start);

function start(){
    //getElementsByTagName
    //jQuery();==>jQuery.fn.init
    $(document).click(function(){
        $(":focus").css("background","red");
    });
}
</script>
<input />
<h1>1</h1>
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
